1. Field of the Invention
This invention relates generally to a corona igniter for emitting a radio frequency electric field to ionize a fuel-air mixture and provide a corona discharge, and a method of forming the corona igniter.
2. Related Art
Corona discharge ignition systems provide an alternating voltage and current, reversing high and low potential electrodes in rapid succession which makes arc formation difficult and enhances the formation of corona discharge. The system includes a corona igniter with a central electrode charged to a high radio frequency voltage potential and creating a strong radio frequency electric field in a combustion chamber. The electric field causes a portion of a mixture of fuel and air in the combustion chamber to ionize and begin dielectric breakdown, facilitating combustion of the fuel-air mixture. The electric field is preferably controlled so that the fuel-air mixture maintains dielectric properties and corona discharge occurs, also referred to as a non-thermal plasma. The ionized portion of the fuel-air mixture forms a flame front which then becomes self-sustaining and combusts the remaining portion of the fuel-air mixture. Preferably, the electric field is controlled so that the fuel-air mixture does not lose all dielectric properties, which would create a thermal plasma and an electric arc between the electrode and grounded cylinder walls, piston, or other portion of the igniter. An example of a corona discharge ignition system is disclosed in U.S. Pat. No. 6,883,507 to Freen.
The corona igniter typically includes the central electrode formed of an electrically conductive material for receiving the high radio frequency voltage and emitting the radio frequency electric field into the combustion chamber to ionize the fuel-air mixture and provide the corona discharge. An insulator formed of an electrically insulating material surrounds the central electrode and is received in a metal shell. The igniter of the corona discharge ignition system does not include any grounded electrode element intentionally placed in close proximity to a firing end of the central electrode. Rather, the ground is preferably provided by cylinder walls or a piston of the ignition system. An example of a corona igniter is disclosed in U.S. Patent Application Publication No. 2010/0083942 to Lykowski and Hampton.
The corona igniter may be assembled such that the clearance between the components results in small air gaps, for example an air gap between the central electrode and the insulator, and also between the insulator and the shell. These gaps are filled with air and gases from the surrounding manufacturing environment and during operation, gases from the combustion chamber. During use of the corona igniter, when energy is supplied to the central electrode, the electrical potential and the voltage drops significantly across the air gaps, as shown in FIGS. 6 and 7. The significant drop is due to the low relative permittivity of air.
The high voltage drop across the air gaps and the spike in electric field strength at the gaps tends to ionize the air in the gaps leading to significant energy loss at the firing end of the igniter. In addition, the ionized air in the gaps is prone to migrating toward the central electrode firing end, forming a conductive path across the insulator to the shell or the cylinder head, and reducing the effectiveness of the corona discharge at the central electrode firing end. The conductive path across the insulator may lead to arcing between those components, which is oftentimes undesired and reduces the quality of ignition at the central electrode firing end.